Credit: © 2008 AAAS

Although they have been around for the past hundred years, ionic salts with low melting points (room-temperature ionic liquids or RTILs) have sparked a great deal of interest over the past decade as greener alternatives to organic solvents. Their properties are easily tunable by judicious choice of ions, but their behaviour, governed by a combination of strong ionic interactions in the absence of screening solvent molecules, is still little understood because of a lack of high-resolution structural information.

Interactions with solid surfaces, in particular, are crucial for applications such as catalysis, batteries or fuel and solar cells. Harald Reichert, at the Max-Planck-Institut für Metallforschung in Stuttgart, and co-workers have now determined1 the structure of three RTILs in contact with a solid sapphire (aluminium oxide) substrate.

The interface was illuminated with a high-energy X-ray beam at various temperatures. Measurements of the resulting reflectivity, combined with computational modelling, enabled the team to determine the electron-density profile across the interface. All three RTILs assembled in clearly separated, alternate cationic and anionic layers near the surface. The formation of layers, which merge in the bulk ionic liquid, was triggered by the sapphire surface becoming negatively charged under the X-ray beam. Although this organization of ions with like charge into layers is counterintuitive, it is thought to be a generic feature of RTILs at a charged surface.